1. Field of the Invention
This invention relates to the chemical synthesis of electrically conducting polymers. In particular, the invention relates to emulsion polymerization of aniline, substituted anilines and mixtures thereof for making substituted or unsubstituted conductive homopolymers and co-polymers of aniline, in the presence of polar and non-polar or weakly polar solvents, and functionalized protonic acids, which perform as surfactants and as protonating agents (dopants) for the resulting electrically conducting polymer. Another aspect of this invention relates to methods for making polyanilines of controlled, highly crystalline and highly oriented morphologies. Yet another aspect relates to methods for making polyanilines of controlled molecular weight.
2. Prior Art
Polymers of aniline, or polyaniline are considered to be attractive as conducting polymers because of excellent stability in comparison with other conjugated polymeric systems. Polyaniline is also of special interest because the electrical properties of polyaniline can be reversibly controlled both by charge-transfer doping and by protonation. Thus, a wide range of associated electrical, electrochemical, and optical properties, coupled with excellent environmental stability, make polyaniline a useful electronic material in a wide variety of technologically important applications. Disclosures of polyaniline and its use can be found in numerous publications and patents. Green, A. G., and Woodhead, A. E., "Aniline-black and Allied Compounds, Part 1," J. Chem. Soc. 101 (1912) 1117; Kobayashi, et al., "Electrochemical Reactions . . . of Polyaniline Film-Coated Electrodes," J. Electroanal. Chem., 177 (1984) 281-91; and U.S. Pat. Nos. 3,963,498; 4,025,463 and 4983,322.
Polyaniline can be synthesized from monomers by both electrochemical and chemicals oxidative polymerization methods. Electrochemical oxidation utilizes an electrochemical charge transfer reaction, whereas chemical oxidation occurs by means of chemical reaction with an appropriate oxidizing agent. Considerable effort has been devoted to development of relationships between the synthetic conditions for electrochemical polymerization and the resulting properties of polyanilines (W.-S. Huang, B. D. Humphrey, A. G. MacDiarmid, J. Chem. Soc. Faraday Trans., 82 (1986) 2385; E. M. Genies, A. A. Syed, C. Tsintavis, Mol. Cryst. Liq. Cryst., 121 (1985) 181).
The chemical oxidative polymerization of anilines is particularly important since this mode of synthesis is the most feasible for large-scale production of polyaniline. The chemical oxidative polymerization of aniline routinely is carried out in acidic aqueous solutions. Recently, Pron et al. (A. Pron, F. Genoud, C. Mernardo, M. Nechtschein, Synth. Metals, 24 (1988) 193) compared the electrical conductivity and the reaction yield of polyaniline, polymerized with four different oxidizing agents and at different aniline/oxidant ratios. These authors concluded that the redox potential of the oxidizing agents is not a dominant parameter in the aqueous chemical polymerization of aniline, because most oxidizing agents gave similar results. Cao et al. (Y. Cao, A. Andreatta, A. J. Heeger, P. Smith, Polymer, 30 (1989) 2305) determined optimal reaction conditions for the chemical polymerization of aniline in acidic aqueous solutions as a function of a wide variety of synthesis parameters, such as pH, relative concentration of reactants, polymerization temperature and time. Armes et al. (S. P. Armes, J. F. Miller, Synth. Metals, 22 (1988) 385) studied the polymerization of aniline at 20.degree. C. using ammonium peroxysulfate as oxidant. In their study of the effect of the initial oxidant/monomer molar ratio. Armes et al. concluded that the conductivity, yield, elemental composition and degree of oxidation of the resulting polyaniline are essentially independent of the initial oxidant/monomer molar ratio when its value was below 1.15. Asturias et al. (G. E. Asturias, A. G. MacDiarmid, A. J. Epstein, ICSM '88, Synth. Metals, 29 (1989) E157) investigated the influence of the polymerization atmosphere (air or argon) on the degree of oxidation of chemically prepared polyaniline, using (NH.sub.4).sub.2 S.sub.2 O.sub.8 as an oxidant in acidic aqueous solutions.
Traditionally, and in the above cited references, aniline is chemically polymerized in an aqueous medium to which an oxidant is added. Sometimes, in addition to the above ingredients, a protonic acid is added to the aqueous polymerization mixture that renders the final polyaniline conductive (A. G. MacDiarmid, J.-C. Chiang, W. Huang, B. D. Humphry, N. L. D. Somasiri, Mol. Cryst. Liq. Cryst. 125 (1985) 309-318; J.-C. Chiang, A. G. McDiarmid, Synth. Metals, 13 (1986) 193-205; W. W. Kocke, G. E. Wnekand Y. Wei., J. Phys. Chem. (1987) 5813-5818; Jap. Pat. No. 63-178442, U.S. Pat. No. 5,069,820).
A number of technological disadvantages are associated with the currently employed acidic aqueous chemical synthesis. Generally, under the chemical polymerization conditions described in the above references, the polymer is obtained in irregular, powdrous form having moderate degrees of crystallinity and lacking macroscopic molecular orientation. The shape of the precipitating polyaniline particles commonly is difficult to control, and most often is globular, and not fibrillar. The formation of fibrillar polyaniline particles in acidic aqueous chemical polymerizations has been disclosed, but only in polymerizations to which additional polymeric species, such as poly(ethyleneoxide) or poly(acrylic acid) have been added (B. Vincent, J. Waterson, J. Chem. Soc. Chem. Commun. (1990) 683; J.-M. Siu, S. C. Yang, J. Chem. Soc., Chem. Commun. (1991) 1529. These methods include the addition of polymers to the aniline polymerization, which is uneconomical, and may have unwanted effects. For example, the addition of poly(ethyleneoxide) results in materials of low thermal stability which is undesirable in many applications. Therefore, the added poly(ethyleneoxide) may have to be removed.
Generally, the formation of fibrillar morphologies are highly desirable in conductive polymers, because such "whiskers" may have enhanced order and superior electrical conductivity over randomly oriented and less crystalline polyaniline particles. In addition, a high length/diameter ratio (known as the aspect ratio) of conducting particles, generally, is of extreme importance in lowering the percolation threshold of electrical conductivity (the minimum % v/v of electrically conductive material in total material at which the material conducts electricity) in blends with non-conducting polymers. This is a major economic advantage. For example, when spherical conductive particles are added to an insulating matrix, a volume fraction of spherical particles greater than about 0.17 is required to impart electrical conductivity to the composition. By contrast, when whisker- or fibril-shaped conductive particles are added to an insulating matrix, the percolation threshold necessary for electrical conductivity may be reduced from 17% v/v to 1% v/v, or sometimes even lower content, depending on the aspect ratio of the fibrils. Higher aspect ratios permit even lower percolation thresholds.
Thus, conductive particles of high aspect ratios are desirable; and a need exists for economical methods to make solid polyanilines of high crystallinity, controlled particle morphology and ordered molecular orientation.